1. Field of the Invention
The present invention relates to a MOS varactor. More particularly, the present invention relates to a MOS varactor for use in circuits and elements of a millimeter-wave frequency band, which is capable of reducing series resistance and enhancing a Q-factor by using a plurality of island-like gates seated in a well region of a substrate and gate contacts directly over the gates, and a method of fabricating the MOS varactor.
2. Description of the Related Art
In pace with recent general trend to information-orientation, many efforts have been made to achieve high speed and high frequencies in the field of wireless communication for high quality wide-band communication services. In this connection, achievement of high integration, high frequency and low noise characteristics of CMOS (Complementary Metal-Oxide Semiconductor) process-based semiconductor devices for RFIC (Radio Frequency Integrated Circuit) has been raised as a major issue.
A CMOS process-based semiconductor device has been conventionally recognized as core technology for implementation of RFIC of a millimeter wave frequency band (30 GHz to 300 GHz) because of its on-chip ability and excellent merits in the aspect of production costs and power consumption and has been increasingly significant, particularly, with extension of a RFIC application range up to a frequency band of 60 GHz or 77 GHz in recent years.
In the meantime, as an operation frequency of RFIC using the CMOS process-based semiconductor device is increasing, performance of not only active elements such as transistors and so on but also passive elements such as inductors, varactors, capacitors and so on in the semiconductor device acts as a primary factor to have a great effect on the entire operation characteristic of a system.
As one example, a voltage-controlled oscillator (VCO) for generating a carrier frequency of a transceiver of a millimeter wave frequency band has a phase noise as its most important characteristic on which a Q (quality) factor of an LC tank including an inductor and a varactor makes core contribution. In this case, although performance deterioration may be mainly problematic as a Q factor of the inductor decides a Q factor of the entire LC tank in a relatively low frequency band of several GHz, the Q factor of the inductor trends to be relatively enhanced while a Q factor of the varactor trends to be rapidly decreased with increase of an operation frequency. As a result, in the millimeter wave frequency band, the Q factor of the varactor acts as a primary factor to have an effect on the entire performance of the LC tank and VCO.
MOS varactor having a so-called multi-finger structure in which stripe-shaped gate electrodes are interdigitated has been mainly used up to recently due to its compatibility with the CMOS device structure. However, such a structure has a fatal drawback in that it suffers from large parasitic resistance leading a low Q factor thereby degrading the performance of the LC tank and VCO that employ the device.